Laser imagers, in which an image is recorded in a photographic material by laser ray scanning, have become popular as a system for recording an output image from a diagnostic instrument such as X-ray CT or MRI (nuclear magnetic resonance instrument). The laser imager system must record and develop the output image in a shorter period of time to obtain a rapid diagnosis.
Known recording lasers used as laser imagers, are He-Ne and semiconductor lasers.
The wavelength range of rays for exposure with a laser imager is, for example, from 600 to 700 nm with a He-Ne laser and an AlGaP (semiconductor) laser and within the infrared range of semiconductors. Of them, a laser imager operated with exposing rays of from 600 to 700 nm has become most popular, since the rays are within the visible range and therefore an optical system with them may be more easily adjusted than one with infrared rays.
However, a laser imager system must use photographic materials containing visible color sensitizing dyes. Additionally, the materials should be more suited to rapid processing than infrared-sensitive photographic materials, especially with respect to the harmful color retention in materials which have been subjected to rapid processing.
To reduce residual color in processed photographic materials, the amount of the dyes therein may be reduced. In accordance with this method, however, the sensitivity of the materials unfavorably lowers. In another method, color retention is reduced by incorporating an adsorbent in the materials. However, this method is also defective in that it reduces residual color by the substantial substitution of the dyes with added adsorbent on the surfaces of silver halide grains in the materials, with an inevitable lowering of the sensitivity of the materials.
The photographic materials to be used with a laser imager should have various characteristics. In particular, since they are exposed in a short period of time of from 10.sup.-7 to 10.sup.-3 second, they must be highly sensitive.
It is known that the output image from a laser imager has portions having a distinctly different density from that of the areas adjacent to the image. These portions cause unevenness in the developed image when developed by rapid development with an automatic processor. Since this may be avoided by promoting the developability of the photographic material to be used therein, it is necessary that the fluctuation of the sensitivity of the photographic material caused by variation of development time is small.
Recently, high temperature rapid processing of photographic materials has become popular, and the processing time of various photographic materials with an automatic machine has been remarkably shortened. In order to attain rapid processing, a developer is needed which is capable of yielding a sufficient sensitivity in a short period of time. Also needed is a photographic material which has a high rate of development to yield a sufficient density in a short period of time and which may be dried in a short period of time after rinsing. In a method which is generally satisfactorily employed for the purpose of improving the easy driability of photographic materials, a suitable amount of a hardening agent (gelatin crosslinking agent) is previously added to the photographic material in the coating step during its preparation, so that the swelling rate of the emulsion layer and the hydrophilic colloid layer constituting the material so that the water content in the processed photographic material before initiation of drying is reduced during the process of development, fixation and rinsing. In accordance with the method, when a larger amount of a hardening agent is added to the material, the drying time may be shortened. However, in the method, the swelling rate of the layers constituting the material is lowered so that development of the material is delayed, causing depression of sensitivity and hardness or to cause depression of the covering power. In addition to this method, other rapid processing methods are known, for example, as described in JP-A 63-136043 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), in which a developer and a fixer both having substantially no gelatin-hardening effect are used. The method is advantageous in promoting the rate of development and the rate of fixation with the processing solutions, while it delays the rate of drying. Therefore, the method could not be said to be sufficiently effective.
On the other hand, as a method of promoting the rate of development and enhancing covering power, the addition of various additives to silver halides is known. For instance, such additives include polyacrylamides as polymer additives in U.S. Pat. Nos. 3,271,158 and 3,514,289 dextran compounds, as saccharide additives, in U.S. Pat. Nos. 3,063,838 and 3,272,631. However, when these compounds are added to photographic materials in such a amount as to sufficiently satisfy the objects, the driability of the materials and this film strength would defectively lower.
Apart from rapid processability, further elevation of the sensitivity and covering power of photographic materials is a goal which has heretofore been sought in this technical field. When elevation of the sensitivity of photographic materials is realized by enlarging the grain size of the silver halide grains therein, this covering power generally lowers. Therefore, if elevation of the sensitivity of photographic materials could not be attained by the use of silver halide grains having the same grain size or if elevation of the covering power thereof could not be attained by the use of silver halide grains having the same sensitivity, then such a result would be meaningless.
Under these circumstances, photographic materials having a high sensitivity and a high covering power and having a high rate of development with rapid processability are desired as those for popular laser imagers which may be easily subjected to optical adjustment and which have an ordinary exposure range of from 600 to 700 nm.
On the other hand, even though the rate of development of photographic materials could be elevated, delay in fixation of the materials due to excess hardening of them would cause other problems of silver remains, hypo remains and residual color from the sensitizing dyes used, which preclude shortening of the processing time.
Some methods of elevating the activity of processing solutions are also known. For instance, effective are increases in the amounts of the developing agent and the developing aid in a developer, elevation of the pH value of a developer, and elevation of the processing temperature. However, all of these methods have the drawbacks that the time-dependent stability of the processing solutions is lowered and the processed materials are often softened or quite fogged.
In addition, in accordance with the proposed methods in which the processing time of development, fixation and rinsing is shortened, photographic sensitivity is lowered, and silver and hypo residues increase due to depression of the fixing capacity. For spectrally-sensitized photographic materials, these methods have the additional problem of residual color due to the sensitizing dyes therein.